1. Field of the Invention
This invention relates to silicone-based polymer coatings.
2. Discussion of the Related Art
Polymers form interconnected networks of molecularly cross-linked oligomers. Polymer networks having dialkyl siloxane backbones are generally known as silicones. Silicones are widely used as coatings and sealants for glass, ceramic, and metal surfaces. Many silicone coatings are transparent to visible and near ultraviolet (UV) light. The transparency of silicones makes these coatings useful for protecting optical surfaces that transmit light.
Conventional silicones are weak and easily tear in response to mechanical stresses, because the coatings have low cross-link densities. The resistance of some conventional silicone coatings to tearing has been increased by adding metal oxide or silica particles to the coatings. Unfortunately, the presence of such particles in the coatings also decreases their transparency and thus, makes these coatings less useful for protecting optical surfaces. The added oxide or silica particles also damage some surfaces that the coatings are used to cover. For example, these particles can scratch optical fibers and the scratches lead to an increase in fiber breakage.
Vinyl addition silicones lack functional groups that readily adhere to silica-glass or metal surfaces and lack an intrinsic polarizablility that could otherwise promote adherence. These properties limit the ability of those silicones to adhere to surfaces with bound hydroxyl groups, because the coatings do not molecularly or electro-statically bind to the surfaces. The inability of those silicones to strongly adhere to underlying surfaces reduces the durability of silicone coatings and enables water to undercut the coatings and thereby promote coating detachment.
In one aspect, the invention features a process for forming a composition. The process includes providing a liquid mixture that includes silicone oligomers and alkyl silsesquioxane oligomers and curing the liquid mixture to form a composition of first and second polymers. The first polymer includes a cross-linked silicone network formed in part of the silicone oligomers. The second polymer includes the alkyl silsesquioxane oligomers cross-linked by siloxane bonds.
In another aspect, the invention features a composition including first and second polymers. The first polymer includes a first network of cross-linked silicone oligomers. The second polymer includes a second network of alkyl silsesquioxane oligomers crosslinked by siloxane bonds.
In another aspect, the invention features an optical router having at least three optical ports. One of the ports optically couples to an optical conduit that includes a diffraction grating and a protective coating. The coating is formed of a composition of first and second polymers that includes silicones and siloxane cross-linked alkyl silsesquioxane oligomers, respectively.
In another aspect, the invention features a process for coating an article with a silicone. The process includes providing a liquid mixture including silicone oligomers and silsesquioxane oligomers, coating a surface of the article with the liquid mixture, and curing the coating on the surface to form a composition of first and second polymers. The first polymer includes the silicone oligomers, which are cross-linked to form another network. The second polymer includes the alkyl silsesquioxane oligomers cross-linked by siloxane bonds to form a network.